1. Field of the Invention
The present invention relates to ultrasonic sensors used as, for example, obstacle detection sensors, such as car-mounted rear sonar devices and corner sensors.
2. Description of the Related Art
Ultrasonic sensors capable of transmitting and receiving ultrasonic waves have been used as obstacle detection sensors, such as car-mounted rear sonar devices. As shown in FIG. 15, such an ultrasonic sensor includes a cylindrical case 100 with a bottom and a piezoelectric element 101 bonded to the inner side of the bottom portion, such that the bottom portion of the case 100 functions as a vibrating surface. A terminal holder 104 that holds metallic terminals 102 and 103 is fixed inside the case 100, and a viscoelastic filler 105 is disposed in a recessed portion formed by the terminal holder 104 and the sidewall of the case 100. A sound absorber 106 is disposed at the inner side (adjacent to the piezoelectric element) of the terminal holder 104. Spring terminals 107 and 108 are attached to the metallic terminals 102 and 103, respectively. The spring terminal 107 is in contact with an electrode on the rear surface of the piezoelectric element 101, and the spring terminal 107 is in contact with the inner surface of the case 100. When a voltage is applied to the piezoelectric element 101, the piezoelectric element 101 is induced to vibrate in radial directions or in the thickness direction thereof, and the vibrating surface of the case 100 vibrates so as to emit ultrasonic waves into the air. Once the case 100 resonates, the vibration continues to some extent even when electrical signals to the piezoelectric element 101 are cut off. This phenomenon is referred to as reverberation. When the reverberation continues for a long period of time, sound waves reflected from target objects cannot be accurately detected because the sound waves are hidden in the reverberation signals, and a resolution of minimum distances to detected obstacles cannot be improved.
To reduce the reverberation, Japanese Unexamined Patent Application Publication No. 2004-146879 discloses a method for reducing variations in reverberation characteristics by setting the hardness of a sealing silicone resin disposed inside a case in a predetermined range. However, the physical properties of such a hardened body of silicone resin vary widely, and it is difficult to reduce variations among manufactured products. Therefore, it is very difficult to control the manufacturing process, and the reverberation characteristics may in fact be impaired and the characteristics may vary more widely when the manufacturing process is not closely controlled.
Japanese Unexamined Patent Application Publication No. 2-36700 discloses an ultrasonic sensor that reduces residual vibration by providing a vibration isolator on a piezoelectric element and the entire inner surface of a cylindrical case and by providing an elastic member inside the cylindrical case so as to prevent resonance of the air inside the case. In this structure, a terminal plate is fixed to an opening of the cylindrical case, and the case and the terminal plate vibrate in an integrated manner. Thus, the vibration of the side surface of the case cannot be sufficiently reduced, and the reverberation cannot be sufficiently reduced.
Japanese Unexamined Patent Application Publication No. 2004-343660 discloses an ultrasonic sensor including a cylindrical case with a bottom, the sidewall portion of the case having a double structure including an inner peripheral wall and an outer peripheral wall, and an elastic body enclosed in a hollow portion between the inner peripheral wall and the outer peripheral wall. This ultrasonic sensor has an effect of preventing vibration of the sidewall portion of the case causing reverberation from being amplified by reducing mutual interference between the vibration of a bumper of a vehicle body and the vibration of the sidewall portion of the case causing reverberation using a damping effect of the elastic body enclosed in the hollow portion of the sidewall portion of the case. However, the ultrasonic sensor cannot effectively reduce the vibration transmitted to the sidewall portion of the case and causing reverberation itself.